Law enforcement departments across the country use the marijuana holiday as a way to build their followers and soften their image on social media.

April 20 has long been the unofficial national holiday for pot-smoking in America. It's also become a chance for police to get some jollies and simultaneously build their social media followers.

Lure them in with jokes, the thinking seems to be, and inform them about serious matters later.

Last year, the police department in Wyoming, Minn., tweeted about its "undercover #420 operations." The post featured a photo of an officer standing with a butterfly net, ready to catch stoners who fall for their trap of video games and junk food munchies.

It was retweeted more than 170,000 times, receiving a total of 29 million impressions. Not bad for the social media account of a police force with 10 officers.

"The approach we've taken is interjecting some personality into our social media page," says Wyoming Police Chief Paul Hoppe. "It's shown to be effective in our ability to reach our audience."

The police in Wyoming, who serve a community of 8,000, now have nearly 28,000 followers on Twitter.

They followed up their viral weed trap tweet with posts about substance abuse. For parents who may be concerned about whether their kids might be using the drug, 4/20 is perhaps the best occasion to check, says Chief Hoppe.

"If we entertain them with humor, and then when we have a mass emergency, that audience is already seeing our information and will share it as well," says Anthony Greiter, an officer for the Iowa State University police.

That department has 32,000 Twitter followers on a campus with 36,000 students. The police account for the similarly-sized University of Iowa, which features sober posts about suspects and tornado drills, has fewer than 11,000 followers.

Another benefit to cops getting casual on social media: Wyoming residents sometimes tell officers that their lighter tone has made them more approachable.

"Part of what we're trying to do is humanize law enforcement a little bit," says Chief Hoppe. "When we take the tone of an institution, it makes it difficult to resonate with individuals."

It's the nature of social media to be less stiff than older, more formal modes of written communication, says Albert Meijer, a professor of public innovation at the Utrecht School of Governance in the Netherlands who has studied police use of social media.

"These messages are sometimes closer to the way police officers speak to citizens on the street," Meijer says. "They do this to reach a large audience and make an impact."

But there's a limit to how casual police want to be.

If a joke appears anywhere near the line of being inappropriate, Greiter will make sure the chief or other higher-ups approve it. He keeps a note on his desk that reads, "Think twice, tweet once."

For some departments, the jokes don't end on April 20.

After tweeting a GIF of talk show host Conan O'Brien sarcastically slow-clapping 4/20's arrival, the Dixon, Ill., Police Department later in the summer posted behind-the-scenes footage of a "raid" on a backyard pool party. The officers were softening up their targets with a barrage of water balloons.